In electronics, there is an ongoing need to improve the efficiency and bandwidth of amplifier circuits. One method for improving the efficiency of an amplifier was invented in 1934 by William H. Doherty. A typical Doherty amplifier is shown in FIG. 1 and has a class-AB main amplifying stage 110 in parallel with a class-C auxiliary amplifier stage 120. The Doherty amplifier receives an input signal from an off-chip source that controls the biasing provided to the amplifier stages using a biasing control circuit. An input signal is split evenly to drive the two amplifiers, and a combining network 102 sums output signals from the main and auxiliary stages and corrects for phase differences between them. During periods of lower signal power levels, the main stage efficiently amplifies the input signal and the auxiliary stage remains off. In this mode, the main amplifier dynamic load impedance is about two times higher than the optimum power match. During higher power signal peaks, main stage 110 approaches compression but remains operating while auxiliary stage 120 also turns on and transforms the dynamic load of both amplifiers to their optimum power match. This increases the overall efficiency dynamic range (which is the input power range over which efficiency remains high) by about 6 dB.
In a typical Doherty amplifier assembly, the class-AB main amplifying stage 110 and the class-C auxiliary amplifier stage 120 are manufactured on integrated circuits having different operating parameters. In a prior art embodiment, an off-chip power detector and bias control circuit 130 is used to provide bias for the class-AB main amplifier 110 and the class-C auxiliary amplifier 120. The same input biasing voltage is provided to each biasing control circuit. In turn, the individual integrated circuits are designed to produce the desired amplification of the various amplification stages 110, 120 using this common biasing voltage. However, the use of an off-chip detector and control solution adds complexity to the design. Thus, a need exists for a Doherty amplifier design that utilizes the same integrated power detection and bias control circuits in both the main amplifying stage and the auxiliary amplifier stage. Furthermore, a need exists for the main amplifier and the auxiliary amplifier to be created using the same MMIC design.